IMAGINE spending four months upside down, half starved, babysitting a brood of tiny eggs under a leaf. Parenting is tough for daddy longlegs living in the Brazilian Atlantic forest, but their efforts aren’t in vain. Having a parent to watch over the eggs makes a huge difference to their survival; without one, a third of clutches are eaten.

Although we humans no longer need to protect our progeny from hungry predators, we are accustomed to thinking that parenting is crucial if offspring are to survive and thrive. Yet, among the myriad organisms on our planet, this is rare. What’s more, in those species that do care for their young there is a strong bias towards females doing all the work. That makes the daddy longlegs a real oddity – it is among the few examples where males alone raise the young.

When it comes to parenting, why do some males buck the trend and take the lion’s share? This question has puzzled scientists from Charles Darwin onwards. They have come up with some ingenious theories, but as we have learned more about nature’s devoted dads, several have fallen by the wayside. We do know that male parental care has evolved independently many times, in a diverse array of animals. Interestingly, exclusive care by dads is clustered in some branches of life’s evolutionary tree, including certain fish and frogs. Each species may have its own story, but the example of the daddy longlegs highlights a surprising common thread. While babysitting, daddy longlegs fathers can receive the amorous attentions – and eggs – of up to …